The method shows promise for many applications, including energy storage – ScienceDaily


A research team, led by chemists from Northwestern University, has made a breakthrough in surface science by introducing a new active adsorption mechanism. Such adsorption-based phenomena, in which molecules are attracted to a solid surface, are essential for today’s catalysts, energy storage and environmental remediation.

Research demonstrates how man-made molecular machines – fully synthetic molecular components that produce machine-like movements – grafted onto surfaces can be used to actively recruit molecules to these surfaces at very high concentrations, thereby storing significant amounts of ‘energy.

The new adsorption mechanism, called mecansorption, results from out-of-equilibrium pumping to form mechanical bonds between the adsorbent (the surface) and the adsorbate (the molecules). Details of the study, titled “Active mechanisorption Driven by pumping cassettes”, will be published online October 21 in the journal Science.

The mechanism uses redox (i.e. reduction followed by oxidation) and acid-base chemistry to adsorb and desorb a fleet of rings with precision on and off the surface of a metal structure -organic (MOF) two-dimensional solid state. In the study, the molecules brought to the surface were rings, but it is expected that the approach could be generalized to include many other molecules by functionalizing the rings to begin with.

“The importance of this research lies in the fact that it is the first major fundamental advance in surface chemistry since physisorption and chemisorption – two phenomena based on equilibrium – were on the agenda. in the 1930s, “said Northwestern’s Sir Fraser Stoddart, who received the 2016 Nobel Prize in Chemistry for his work involving the design and synthesis of molecular machines.

Stoddart, professor of chemistry on the board of trustees of the Weinberg College of Arts and Sciences, is the corresponding author of the survey, together with professor of the University of Maine Dean Astumian, theorist in the department of physics and astronomy, and Omar Farha, an MOF chemistry expert and chemistry professor at Northwestern. Liang Feng and Yunyan Qiu, postdoctoral fellows at the Stoddart lab, are the co-first authors of the article.

“There is good reason to believe that the concept of mecansorption will grab the attention of textbooks someday,” Stoddart said. “If chemists can figure out how mecansorption can be incorporated into active structures, the storage of gases like hydrogen, carbon dioxide, and methane will enter a whole new world and become a whole different ball game.”

Research illustrates the synergy that results from combining theory with experience. The idea for a pumping cassette arose from Astumian’s examination of the effects of oscillating electric fields on membrane-bound enzymes. (A pumping cassette can be likened to a “valley” whose “floor” can be moved up and down surrounded by two “mountain passes” whose heights can be raised and lowered so that the molecules are forced to move up and down. move in one direction.) the molecular contraption was synthetically implemented in Stoddart’s lab using rotaxanes – long dumbbell-shaped molecules – terminated at one or both ends with a recognition site for them. rings surrounded by two groups to provide kinetic barriers between the mass where the rings swim in solution and polymer chains where the rings are collected one at a time after each redox cycle. It is important to note that these barrier-forming groups may be designed to respond differently to changes in their environment. These pump cassettes can be incorporated on many types of polymer chains, giving rise to many possible applications.

Mechanisorption has important implications for the storage and controlled release of many different molecules. This work focuses on the recruitment of annular molecules to surfaces, but it is anticipated that these rings can be functionalized to bring many different types of molecules in high concentration to surfaces.

“The mechanism of mecansorption has some characteristics in common with aerosol cans,” said Stoddart, “where different materials are stored at high pressure and then released by pressing a trigger. Mechanized substances, however, remain in mechanical equilibrium even when they are released. are packed away from thermodynamic equilibrium.The triggered release mechanism involves diffusion only, a process which, although apparently macroscopically slow, is remarkably fast in these systems.

Astumian at the University of Maine stresses that research is also important to understanding one of chemistry’s deepest questions. “What are the principles by which simple matter becomes complex? he said. “A key point is that, while thermodynamics determines the most likely structures close to equilibrium, kinetics play the dominant role in the selection of structures far from equilibrium.”

In the 1930s, Irving Langmuir and John Lennard-Jones observed that adsorbates interact with surfaces via van der Waals interactions (physisorption) and / or electronic interactions (chemisorption). Adsorption is generally considered to be a passive process in which the adsorbate moves from an area of ​​high concentration to an area of ​​low concentration, so that the concentration of the surface adsorbate always changes towards the direction of equilibrium. . In the Northwestern study, however, researchers demonstrate that active adsorption can be achieved using man-made molecular machines.

“The potential utility of mecansorption in technology, such as chemical capacitors, will provide a whole new way to store and manipulate energy, information and matter on surfaces never imagined before” , said co-first author Feng. “The advent of mechanical bonding has major repercussions on both chemistry and materials science. With a little more time, the general field of sorption will undergo a profound change after nearly a century in which physisorption and chemisorption have dominated surfaces and interfaces. science.”

Co-lead author Qiu added, “This research is the first example of using artificial molecular pumps to actively recruit and adsorb molecules to solid surfaces and opens the door to the exploitation of artificial molecular machines on solid surfaces. ‘a range of functional materials, from zeolites and metal oxides to polymer networks and micellar nanoparticles. “

Experts familiar with the work but not involved in the study noted the importance of the research and its potential applications.

“The extraction of chemicals from solution in and on solids and surfaces underlies the sequestration of wastes and pollutants, the recovery of precious metals, heterogeneous catalysis, many forms of chemical and biological analysis and the separation science, and many other technologies, ”said David Leigh. , Royal Society Research Professor at the University of Manchester in the United Kingdom.

“Until now, there was no way to actively drive such processes, but the use of molecular machines alters this, through a mechanism the Northwestern team calls ‘mechanosorption’,” he said. he declared. “Miniaturization has driven advancements in technology through the ages, and the use of molecular-sized machines – molecular nanotechnology – to fuel adsorption will surely continue this trend.”

Jonathan Sessler, Doherty-Welch Chair in Chemistry at the University of Texas at Austin, said of the research: “This is a game-changer. This opens a new chapter in the very important and generally energy-intensive field of the author team has shown for the first time that it is possible to use mechanically linked pumping strategies to concentrate highly charged species against a Coulomb gradient.

“The use of electrochemical methods to drive this chemical process out of equilibrium opens up the possibility of direct use of solar energy to enable separations,” Sessler said. “Ultimately, this approach could enable cost-effective capture, remediation and purification of key industrial targets, such as hydrocarbons, carbon dioxide and micropollutants. asymmetric and non-racemic thread entities could allow chiral separations. The opportunities seem almost endless. “


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